Rapid solidification of martensitic stainless steel atomized droplets

The microstructure of martensitic stainless steel powders produced by inert gas atomization was investigated. Depending upon the powder particle size, the microstructure was found to exhibit a cellular, dendritic, or martensitic morphology. Relationships between the microstructure scale and the particle diameter were identified. It was found that at a critical particle diameter of 25 to 30 μm, the structure changed from cellular/dendritic (96.5 vol pct bcc and 3.5 vol pct fcc) to martensite. The solidification path of the powder particles below and above 25 to 30 μm in size was considered. High-temperature X-ray diffraction (HTXRD) measurements revealed that there is a delay in the appearance of the fcc phase for the small particle size. The delay in the appearance of the fcc phase is a result of different nucleation sites for the fcc phase between the large and the small particle size.     

Deformation of martensite in a polycrystalline Cu-Zn-AI alloy

The deformation morphologies and mechanisms in polycrystalline martensitic Cu-Zn-AI samples have been examined. A variety of deformation morphologies are exhibited simultaneously in moderately cold-worked polycrystalline samples. The mechanisms by which these are developed have been ascertained, and include variant-variant coalescence, stress-induced martensite-to-martensite transformation, injection of foreign variants to plate groups, internal twinning, and slip. The crystallographic features of the various deformation morphologies have been completely characterized using transmission electron microscopy. Transformation peaks monitored by differential scanning calorimetry showed tails on the completion side, and diminished peak size, for cold worked specimens. These effects are correlated with observed microstructural features.     

铝合金中镁的快速测定

所介绍的铝合金中镁的快速测定,采用DDTC、盐酸羟胺、三乙醇胺联合掩蔽剂掩蔽大量的铝及少量的铜铁锰离子,在PH10以铬黑T为指示剂,用EDTA滴至溶液由紫红色变为纯兰色为终点。方法简单,快速,准确度高,适合于铝合金中镁含量的测定,是一种较好的铝合金中测定镁的方法。     

Rapid solidification of a droplet-processed stainless steel

Individual powder particles of a droplet-processed and rapidly solidified 303 stainless steel are characterized in terms of microstructure and composition variations within the solidification structure using scanning transmission electron microscopy (STEM). Fcc is found to be the crystallization phase in powder particles larger than about 70 micron diameter, and bcc is the crystallization phase in the smaller powder particles. An important difference in partitioning behavior between these two crystal structures of this alloy is found in that solute elements are more completely trapped in the bcc structures. Massive solidification of bcc structures is found to produce supersaturated solid solutions which are retained to ambient temperatures in the smallest powder particles. Calculated liquid-to-crystal nucleation temperatures for fcc and bcc show a tendency for bcc nucleation at the large liquid supercoolings which are likely to occur in smaller droplets. The importance of small droplet sizes in rapid solidification processes is stressed. Formerly with Massachusetts Institute of Technology, Cambridge, MA.     

Rapid solidification processing of a Mg-Li-Si-Ag alloy

A Mg-13Li-4Si-lAg (wt pct) alloy with improved ductility and thermal stability was developedvia the rapid solidification (RS) processing technique. Silicon was added to the alloy as the third alloying element in order to form a thermally stable intermetallic dispersoid phase required for improved mechanical properties at ambient and elevated temperatures. The microstructure of the as-spun and heat-treated alloy was characterized using differential scanning calorimetry (DSC), X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Microhardness measurements were conducted on as-spun and heat-treated alloy in order to obtain qualitative prop-erty data and to investigate the extent of the degradation of properties at elevated temperatures. It was found that the melt-spun Mg-Li alloy possessed a microstructure consisting of a fine dispersion of Mg₂Si phase in a fine-grained body-centered cubic (bcc) Mg-Li solid solution, resulting in the desired improvements in thermal stability and mechanical properties. Formerly Graduate Student, Department of Materials Science and Engineering, University of California     

Rapid solidification characteristics in melt spinning a Ni-base superalloy

The solidification kinetics involved in the process of melt spinning a Ni-base superalloy have been characterized. Through a correlation of ribbon thickness to melt puddle residence time, it was found that the solidification front velocity,V, is typically about 100 mm per second at the ribbon surface not in contact with the spinning wheel. The rate of solidification varies within the ribbon, increasing with decreasing distance,S, from the wheel-contact surface asV = 3.65S ^-1. Ribbon microstructure and texture characteristics are discussed in light of this kinetics result. The thickness-vs-time correlation was further analyzed to yield information about thermal history during ribbon formation. These thermal results are generally consistent with those deduced from dendrite arm spacing measurements. Formerly with General Electric Research and Development Center.     

The stabilization of martensite in Cu-Zn-AI shape memory alloys

The martensitic transformation temperature in shape memory alloys can be affected differently by aging above and below the transformation temperature. Under such circumstances the normally reversible transformation can be prevented and the martensite structure “stabilized”. This effect has been studied using electron microscopy, differential scanning calorimetry, and mechanical testing. Evidence is given of an apparently martensitic high temperature transformation, and a careful comparison is made of the stabilized and unstabilized states of the alloy. Three possible models for stabilization are considered in the light of the results obtained, and it is concluded that no single mechanism can be responsible for all the phenomena observed. Formerly with the Department of Metallurgy and Materials Science, University of Cambridge, England     

Nucleation and growth of bainite crystals in Cu-Zn-AI alloys

The bainitic transformation in Cu-Zn-Al alloys is known to have both martensitic and diffusion-controlled natures. To study the relation between these two reactions involved in the bainitic transformation, the present authors have performed a series of investigations mainly by optical and electron microscopy. A mechanism whereby directional diffusion of solute atoms stimulates the nucleation and growth of bainite is proposed based upon these investigations. The mechanism is supported by an additional experiment in which a local solute concentration in the matrix crystal is directly measured around the bainite plates using a special analytical electron microscope. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Rapid melting and solidification of a surface layer

Metallurgical and Materials Transactions B - A one-dimensional computer heat flow model is used to investigate the effect of high intensity heat fluxes,e.g. those achieved via continuous CO2 laser...     

Rapid melting and solidification of a surface layer

A one-dimensional computer heat flow model is used to investigate the effect of high intensity heat fluxes,e.g. those achieved via continuous CO2 laser radiation, on the important surface layer melting and subsequent solidification variables of three substrate materials: aluminum, iron, and nickel. Temperature profilesvs time, melting, and solidification interface velocities, heating, and cooling rates in the surface layers of the three metals are calculated. Results are presented in a general form to permit determination of these variables for large ranges of absorbed heat fluxes and times. General trends established show that temperature gradients in the liquid and solid phases and interface velocities are directly proportional to the absorbed heat flux, whereas melt depth is inversely proportional to the absorbed heat flux. Average cooling rates comparable to splat cooling can be achieved by increasing the heat flux and reducing the dwell time of the incident radiation. An order of magnitude increase in the absorbed heat flux results in a corresponding two orders of magnitude increase in average cooling rates in the liquid during solidification of crystalline and noncrystalline structures. Formerly Research Associate, Formerly Research Associate, Formerly Research Associate,     

Nanocomposites of Aluminum Alloys by Rapid Solidification Processing

Aluminium alloys reinforced with transition metal aluminide (Al₃Ti, Al₃Fe, Al₃Ni, etc.) particles possess high specific strength both at ambient and elevated temperature. The improved strength of these alloys are the results of slower coarsening rate of the intermetallic particles due to low diffusivity of the transition metals in aluminium. However, the strength can be enhanced further by refining the microstructure of the alloys to nanometer range. The authors have successfully attempted two important non-equilibrium processing techniques i.e. rapid solidification processing (RSP) and mechanical alloying for the refinement of the microstructure in various aluminium alloys. In this report, authors present a short review of their work on RSP of Al?CTi and Al?CFe alloys to produce nanocomposites.     

Natural convection transients and their effects in unidirectional solidification

A formulation is given and computed solutions are presented for transient solidification accompanied by natural convection in a vertical slot. It was found that appreciable fluid velocities may be produced by natural convection, the values of which could be comparable to the terminal rising velocities of typical nonmetallic inclusions. The simplifying assumptions made limit the validity of the solutions to systems where GrPr < 500,i.e., to narrow slots or to low values of the superheat; nonetheless, the results should be indicative of the effects of convection at much higher values of GrPr. Formerly Post-Doctoral Research Associate, State University of New York at Buffalo     

Rapid melting and solidification of surface due to stationary heat flux

A general two-dimensional computer heat flow model is developed in an oblate spheroidal coordinate system for rapid melting and subsequent solidification of the surface of a semiinfinite solid subjected to a high intensity heat flux over a circular region on its bounding surface. Generalized numerical solutions are presented for an aluminum substrate subjected to both uniform and Gaussian heat flux distributions. Temperature distributions, melt depth and geometry, and melting and solidification interface velocities are calculated as a function of applied heat flux, radius of the circular region, and time. It is shown that the important melting and solidification parameters are a function of the product of the absorbed heat flux, q, and the radius of the circular region, a. General trends established show that melt depth perpendicular to the surface is inversely proportional to the absorbed heat flux for a given temperature at the center of the circular region. Dimensionless temperature distributions and the ratio of liquid-solid interface velocity to absorbed heat flux,R/q, as a function of dimensionless melt depth remain the same if the productqa is kept constant, whileq anda are varied. For a given total power absorbed melting and solidification parameters are compared for uniform and Gaussian heat flux distributions. For a given temperature at the center of the circular region both melt depth and width are smaller for the Gaussian distribution while temperature gradients and interface velocities are larger. Formerly Graduate Research Assistant, Department of Mechanical and Industrial Engineering, University of Illinois. Formerly Research Associate, Department of Metallurgy and Mining Engineering,University of Illinois. Formerly Professor at the University of Illinois, Urban, IL.     

The effects of applied stress on the martensitic transformation in TiNi

The deformation behavior of TiNi at 20°C has been investigated as a function of composition and of the prior heat treatment. Wide mechanical property variation and significant differences between the effects of tensile and of compressive loading were observed. Under some conditions anelastic behavior, characterized by a broad hysteresis loop, was reproducibly obtained. The effects of heating the deformed materials aboveA _f on the subsequent stress-strain behavior indicate the anomalies observed to be directly related to the martensitic transformation. The effects of stress application on the martensitic transformation are discussed. It is shown that, under some conditions, the stress-assisted transformation structures may be unstable on the removal of the stress. At test temperatures outside theM _f toA _f range, this can result in anelastic behavior. More complex behavior expected at temperaturesM _f < T < A_f, is discussed in some detail. It is shown that both the anelastic behavior and the “shape memory” can be accounted for by the effects of applied stress. It is also shown that the mechanical properties in this temperature range can vary markedly with the prior heat treatment, even at temperatures not normally considered of significance. Though based on observations made on TiNi, the phenomena discussed are inherent in materials undergoing a martensitic transformation over a narrow range of temperatures.     

Influence of training time and temperature on shape memory effect in Cu-Zn-AI alloys

The influence of training temperature on the shape memory effect (SME) for the CuZnAl shape memory alloy (SMA) has been studied. It is found that there exists an optimum upper training temperature resulting in the best SME which is about 353 K for the Cu-26Zn-4Al alloy. The preferential oriented martensitic variants will be formed during cooling, since there are aligned dislocations (resulting in the best two-way memory effect (TWME)) in the parent phase after training between 293 and 353 K. However, the TWME drops gradually due to the generation of dislocation tangles in the parent phase as the training temperature increases to 373 K. Further work on the effect of training time on the SME shows that the shift ofA ^s temperature is little when alloys studied are trained at the optimum temperature for 15 to 40 seconds and there exists a certain training time without any shift ofA ^s temperature after 1000 thermal cyclings.     

Active solute effects on surface ripples in electron-beam welding solidification

Surface rippling on workpieces containing surface-active solutes after solidification, during electron-beam welding or melting, is experimentally and analytically investigated. Most alloys contain one or more surface-active solutes such as O, S, Se, and Te. This leads to surface-tension coefficients being positive at low temperatures. In this work, the accelerating voltage and welding currents of the electron-beam welder used are 50 kV and 10 and 15 mA, respectively, with welding speeds of 20, 30, and 40 mm/s, while the workpieces are different steels containing sulfur of 0.0035, 0.008, and 0.014 wt pct and oxygen of 0.002 wt pct. The average amplitudes of ripples are measured for different beam powers, welding speeds, and sulfur contents. Extending a previous work studying rippling on alloys having negative surface-tension coefficients, the present work predicts average amplitudes of ripples by accounting for heat transfer and fluid flow induced by a positive temperature-dependent surface-tension gradient in the molten pool near the solidification front. It is found for the first time that dimensionless average amplitudes are increased by increasing the product of the Prandtl and Marangoni numbers, the product of the adsorption coefficient and the active solute content, and the elasticity number. The predicted results show good agreement with experimental data.